Method for operating a vehicle powertrain

ABSTRACT

A method of operating a vehicle powertrain, includes: sensing a vehicle velocity; selecting at least two of a plurality of control strategies; activating the at least two control strategies, said two control strategies including: (i) operating the vehicle in a stationary start-stop mode when the vehicle speed is below a first threshold; and (ii) operating the vehicle in a rolling stationary start-stop mode when the vehicle speed is above a second threshold.

The invention relates to a method for operating a motor vehicle having a drive motor which is embodied as an internal combustion engine, and to a drive system of such a motor vehicle.

The drive motor of a motor vehicle serves primarily for generating a driving force which is transmitted to at least one driven wheel in order to generate locomotion of the motor vehicle. The drive motor is embodied here, in particular, as an internal combustion engine, for example as a spark-ignition engine or diesel engine. Since an internal combustion engine already consumes fuel when it is operating without a drive force being generated, various methods have been proposed to reduce the fuel consumption in driving situations in which no driving force is required.

It is therefore known, for example, from WO 201 0/1 21 861 A1, to operate a vehicle as a function of the result of a plausibility check of an automatic speed control function or adaptive speed control function and/or other current vehicle operating data or vehicle state data in a driving mode in which the frictional engagement in the drive train is interrupted and the drive motor is operated at an idling speed or even switched off. DE 102 21 701 A1 describes a “coasting mode”, in which the motor vehicle drives or rolls in the declutched state and the engine speed is approximately or precisely equal to the idling speed; if appropriate the engine can be switched off. The coasting mode is selected if neither the gas pedal nor the brake pedal is actuated; the coasting mode is ended by actuating the brake and/or the gas pedal. However, the specified methods are not optimum in all driving situations.

On the other hand, in order to decelerate a motor vehicle, in particular when traveling downhill, the energy-consuming effect of the engine can be utilized if the engine is being driven by means of the frictional engagement with the driven wheels. Automated methods of this type are known, for example, from DE 603 06 411 T2 and WO 2005/084995 A1. These methods are likewise not suitable for all driving situations which occur, for example, when a motor vehicle is allowed to come to a standstill as a function of the speed.

The drive motor of a motor vehicle is generally also required to supply the electrical systems of the motor vehicle with electrical energy via a generator. In order to ensure the supply of electrical energy during travel even when the drive motor is switched off, a correspondingly adapted energy supply system is necessary which contains, for example, a relatively large battery and a complex controller. This increases costs and weight and results in a higher degree of complexity.

The object of the present invention is to provide a method for operating a motor vehicle having a drive motor which is embodied as an internal combustion engine and a drive system of such a motor vehicle with which the specified disadvantages are as far as possible avoided. In particular, the object of the present invention is to provide such a method and such a drive system in which the fuel consumption is reduced without complex adaptation of the energy supply system being necessary.

This object is achieved by means of a method and by means of a drive system as specified in the independent claims.

According to the invention, a motor vehicle has a drive motor which is embodied as an internal combustion engine. The drive system of the motor vehicle also comprises a drive train for transmitting the driving force generated by the internal combustion engine to at least one driven wheel, the drive train having means for bringing about and/or interrupting the frictional engagement between the drive motor and driven wheel in a controlled fashion, said means being, in particular an automatically actuable clutch. The drive train can also comprise a transmission which can be embodied as a manual transmission or as an automatic transmission.

According to the invention, a respective current velocity of the motor vehicle is sensed. In addition, actuation of at least one speed control means, which may be, in particular, a gas pedal or a brake pedal or else, for example, an operator control element of a speed control system, is sensed. Actuation of the gas pedal and actuation of the brake pedal are preferably sensed. In this way, non-actuation of the speed control means is also sensed, i.e. it is, in particular, also sensed if the driver of the motor vehicle does not actuate the gas pedal and/or the brake pedal.

A driving mode of the motor vehicle is selected automatically on the basis of the sensed velocity and the actuation or non-actuation of the speed control means and, if appropriate, on the basis of further parameters. In this context, when there is a velocity below a maximum speed of a stationary mode, the stationary mode in which the drive motor of the motor vehicle is stopped is selected. In the stationary mode, the frictional engagement between the drive motor and the driven wheel is interrupted, in particular, by opening the automatically actuable clutch; in the case of an automatic transmission with torque converter, the clutch in the drive train can also be closed. The maximum speed of the stationary mode may be, in particular, 0, i.e. the stationary mode is implemented only in the stationary state of the motor vehicle. The maximum speed of the stationary mode can, however, also be different from 0, for example approximately 5 km/h, with the result that even in the case of very slow travel the motor vehicle is in the stationary mode if the drive power of the drive motor is not required for generating or maintaining the locomotion of the motor vehicle. This may be the case irrespective of actuation of a brake pedal.

If the velocity is higher than a minimum speed of a coasting mode, the coasting mode is selected when the speed control means is not actuated. In the coasting mode, the internal combustion engine is operated in the idling mode, i.e. at least approximately at the idling speed; the frictional engagement in the drive train is interrupted here and in particular the clutch which can be actuated automatically is opened. The internal combustion engine can also be operated at least temporarily at a rotational speed which corresponds to the rotational speed of a motor-side drive shaft of a transmission, with the result that a jolt-free transition into a driving mode with closed frictional engagement is made possible. The minimum speed of the coasting mode is higher than the maximum speed of the stationary mode, for example, the minimum speed of the coasting mode is higher than approximately 8 km/h or, preferably higher than approximately 15 km/h.

According to the invention, a rolling mode is selected when there is a velocity above the maximum speed of the stationary mode, but below the minimum speed of the coasting mode, when the speed control means is not actuated, i.e. in particular when the driver actuates neither the gas pedal nor the brake pedal. A minimum speed of the rolling mode is higher than or equal to the maximum speed of the stationary mode, and a maximum speed of the rolling mode is lower than or equal to the minimum speed of the coasting mode. In the rolling mode, the frictional engagement between the drive motor of the motor vehicle and the driven wheel or the driven wheels is interrupted; the drive motor is stopped in the rolling mode.

The method according to the invention can advantageously be used when a motor vehicle is allowed to roll from an initial speed to a relatively low final speed or to a stationary state. The method is, however, also advantageous when traveling downhill from a stationary state or from an initial speed to a relatively high final speed. The method according to the invention can also be advantageously used when rolling to a standstill when traveling downhill or, for example, even in other driving situations in which the gas pedal is not actuated.

Using the rolling mode in a speed range between a maximum speed of the stationary mode and a minimum speed of the coasting mode makes it possible that in a medium speed range the drive motor is switched off entirely while in a relatively high speed range the drive motor is operated in the idling mode.

On the one hand, only a minimum of fuel is consumed as a result; on the other hand, in many cases, the supply of electrical energy is ensured without modification with the result that no complex adaptation of the electrical energy supply system of the motor vehicle is necessary. According to the invention, the motor vehicle is therefore controlled as it comes to a standstill or travels downhill in such a way that even with limited adaptation of the electrical system a mode of driving which is particularly economical in terms of fuel is made possible.

In the stationary mode and/or in the rolling mode an electric starter can advantageously be connected to the internal combustion engine of the motor vehicle, for example by virtue of the fact that a starter clutch is closed or the starter is engaged. As a result the electric starter is already in engagement with the drive motor before switching over into another driving mode occurs, which can be triggered for example by actuation of the gas pedal, so that a faster start of the drive motor is made possible when required.

The maximum speed of the stationary mode may be equal to a minimum speed of the rolling mode above which the rolling mode is selected when the speed control means is not actuated. However, according to one preferred embodiment of the method according to the invention, the minimum speed of the rolling mode is higher than the maximum speed of the stationary mode, and when there is a velocity above the maximum speed of the stationary mode and below the minimum speed of the rolling mode, a creeping mode is selected. In the creeping mode there is frictional engagement between the drive motor and the driven wheel or the driven wheels of the motor vehicle. In this mode the drive motor therefore runs but at a rotational speed which corresponds to the low speed and which, in addition, may also depend on the transmission ratio of a transmission and, if appropriate, on the slip of a torque converter of an automatic transmission and on the position of the gas pedal position. This provides particularly favorable response behavior when opening the throttle.

According to one particularly preferred embodiment of the method according to the invention, a passive deceleration mode is selected when there is a velocity above a maximum speed of the coasting mode and thus also above a maximum speed of the rolling mode when the speed control means is not actuated. In the passive deceleration mode there is frictional engagement between the drive motor and the driven wheel or the driven wheels of the motor vehicle; the supply of fuel to the drive motor is interrupted. The drive motor runs here at a rotational speed above the idling speed. In this mode, the drive motor itself is used to decelerate the motor vehicle. Such a mode is known per se, for example, as “engine braking with overrun fuel cutoff”. The clutch of the electric starter is opened. As a result of the passive deceleration mode, a braking effect can be achieved in a simple and gentle fashion, in particular if the driver does not actuate the gas pedal or the brake pedal.

In a preferred fashion, the speed control means comprise a brake pedal, in which case a further driving mode is selected when the brake pedal is actuated. The further driving mode permits a mode of driving which is particularly economical in terms of fuel even in the case of a deceleration process which is initiated or assisted by actuation of the brake pedal, without complex adaptation of the electrical system of the motor vehicle. If a maximum speed is determined for the coasting mode, this also applies in particular when there is a velocity above this speed.

In a particularly preferred fashion, the further driving mode is an active deceleration mode. In such a driving mode, the frictional engagement between the drive motor of the motor vehicle and the driven wheel or the driven wheels is closed and the supply of fuel to the drive motor is interrupted. This driving mode therefore corresponds to the passive deceleration mode, but in this case the brake pedal is actuated. The active deceleration mode can also be initiated when the brake pedal is actuated and can therefore remain active until a further driving mode is initiated, without the brake pedal being actuated further.

As an alternative to this the further driving mode can also be configured in such a way that the frictional engagement between drive motor and the driven wheel or the driven wheels of the motor vehicle is interrupted and the drive motor is stopped. This corresponds to the rolling mode, but in this case the brake pedal is actuated or has been actuated if the further driving mode is initiated by actuating the brake pedal. In a preferred fashion the starter clutch of the electric starter is closed in this driving mode, with the result that the electric starter is connected to the drive motor, and when a driving force is requested, for example when the gas pedal is actuated, particularly rapid starting of the drive motor is made possible.

Alternatively, in the further driving mode the frictional engagement between the drive motor and the at least one driven wheel of the motor vehicle can be interrupted and the drive motor can be operated in the idling mode, i.e. substantially at the idling speed. This corresponds to the coasting mode, but in this case the brake pedal is actuated at the same time or the brake pedal has been actuated in order to initiate the further driving mode.

As a further alternative, the further driving mode can also be embodied in such a way that the frictional engagement between the drive motor of the motor vehicle and the driven wheel or the driven wheels is interrupted, in particular by opening an automatically actuable clutch in the drive train, and the drive motor is operated at a rotational speed which corresponds to the rotational speed of a motor-side drive shaft of the transmission. In this mode, jolt-free restoration of the frictional engagement or jolt-free closing of the clutch is therefore possible.

Several or all of the specified alternatives can also be implemented successively as a function of the speed, and, if appropriate, further parameters. In particular, before the frictional engagement between the drive motor and the at least one driven wheel is restored, the drive motor can be operated at a rotational speed which corresponds to the rotational speed of a motor-side drive shaft of the transmission. This brings about a jolt-free transition between a driving mode without frictional engagement to a driving mode with frictional engagement between the drive motor and the driven wheel.

An inventive drive system of a motor vehicle comprises a drive motor and a drive train for transmitting a driving force to at least one driven wheel of the motor vehicle, in particular to the two driven wheels, or in the case of all-wheel vehicles, to all four driven wheels. The drive train comprises an automatically actuable clutch for interrupting or closing the frictional engagement between the drive motor and the at least one driven wheel. The drive train can also have a transmission which may be, for example, a manual transmission or an automatic transmission with predefined drive positions or else an automatic continuously variable transmission.

According to the invention, the drive system also comprises a control device for controlling the drive motor and the automatically actuable clutch which is designed to carry out a method as claimed in one of the preceding claims. For this purpose, the control device has, in particular, signal inputs for sensing signals which represent the velocity of the motor vehicle and the actuation of at least one speed control means, for example an gas pedal and/or brake pedal. Such signals are generally sensed for displaying to the driver and/or for controlling other systems and are available to the control device. The control device also comprises processor means for determining a driving mode as a function of the velocity and of actuation of the at least one speed control means as well as control means for actuating the automatically actuable clutch, the internal combustion engine and if appropriate the electric starter and the starter clutch. The control device can also be embodied as part of the motor controller.

The invention will be explained in more detail below by way of example with reference to the drawing, in which:

FIG. 1 shows a collection of driving strategies corresponding to exemplary embodiments of the method according to the invention.

FIG. 1 illustrates four different driving strategies according to which a motor vehicle can be operated according to the invention and the drive system of the motor vehicle can be correspondingly actuated. In this context the current velocity of the motor vehicle is plotted at the top. On the right the different strategies 1 to 4 are illustrated, denoted by the numbers above the corresponding column.

In all the driving strategies, the drive system is actuated at a velocity below a speed v₁ and, in particular, in the stationary state in such a way that a stationary mode (SSS, “Static Start Stop”) is implemented. The stationary mode is defined by an interrupted frictional engagement in the drive train; in the case of an automatic transmission with a torque converter, the frictional engagement can be closed via the torque converter. The internal combustion engine which serves to drive the motor vehicle is stopped in the stationary mode; the starter of the internal combustion engine can be disconnected therefrom by opening the starter clutch. The starter can also be engaged in order to make faster starting possible. If there is a request for driving force by actuating the gas pedal, the internal combustion engine is, if appropriate, started by the starter after the starter clutch has been closed, and the opened clutch in the drive train is closed and/or manual closing of the clutch is made possible.

According to strategy 1, the rolling mode (RSS, “Rolling Start Stop”) is selected starting from a velocity v₁, which corresponds to the maximum speed of the stationary mode and the minimum speed of the rolling mode, if neither the brake pedal nor the gas pedal is actuated. In the rolling mode, the drive system is actuated by the control device in such a way that the frictional engagement in the drive train is interrupted, for example by opening a clutch, and the drive motor is stopped. The starter can be connected to the drive motor in order to start the latter when necessary, for example when the gas pedal is actuated.

Starting from a velocity v₃, which represents the maximum speed of the rolling mode and the minimum speed of the coasting mode, the coasting mode (SAIL) is selected if neither the brake pedal nor the gas pedal is actuated. In the coasting mode, the frictional engagement in the drive train is interrupted, but in contrast to the rolling mode, the drive motor is in operation, in particular at the idling speed. The starter is not connected to the drive motor. The coasting mode can preferably be used up to a maximum speed v₅ of the motor vehicle, but v₅ can also be selected to be lower.

A transition from one driving mode to another can take place by changing the speed, for example, by slowing down when allowing the vehicle to come to a standstill or by increasing the speed when driving downhill. A driving mode can also be ended by actuating the gas pedal or the brake pedal. Likewise, it is possible to end a driving mode by deactivating the economy driving settings, by means of the software or hardware switches, such as start/stop or on/off. Reasons for changing the driving mode can be the switches, as mentioned, passenger comfort and/or air conditioning, the state of charge for the battery, emission control of the engine, current altitude, ambient temperature, engine temperature.

According to strategy 2, the maximum speed v₁ of the stationary mode corresponds to the minimum speed of the creeping mode (CREEP); the maximum speed v₂ of the creeping mode is the minimum speed of the rolling mode. As is apparent from FIG. 1, according to strategy 2, the creeping mode is selected between the minimum speed and the maximum speed of the creeping mode v₁, v₂ if neither the brake pedal nor the gas pedal is actuated. In the creeping mode, the frictional engagement between the drive motor and the driven wheels is closed, the drive motor is operating at a corresponding rotational speed and the starter is disconnected from the drive motor. Above the maximum speed v₂ of the creeping mode the driving strategy 2 corresponds to the driving strategy 1.

According to strategy 3, the coasting mode is used up to a maximum speed v₄. In the case of a velocity which is higher than v₄, a passive deceleration mode (DFSO passive, “Deceleration Fuel Shut Off”) is selected provided that the driver does not actuate the brake pedal or the gas pedal. In the passive deceleration mode, the frictional engagement in the drive train is brought about by closing the corresponding clutch with the result that the drive motor runs at a rotational speed which corresponds to the current velocity and to the respectively engaged gear or the respective driving position, if appropriate, while taking into account the slip of a torque converter of an automatic transmission. The fuel supply to the internal combustion engine is, however, interrupted so that a deceleration effect of the engine occurs, and said effect is used according to the driving strategy 3 to control the coming to a standstill or the driving downhill. The starter is not connected to the drive motor. The passive deceleration mode is used up to the maximum speed v₅ of the motor vehicle. In the case of a velocity below v₄ the driving strategy 3 corresponds to the strategy 1.

As is shown by FIG. 1, the strategy 4 corresponds to the strategy 2 when there is a speed below v₃ and to the strategy 3 when there is a speed above v₃. In this respect, reference is made to the above explanations.

The speeds v₁ to v₅ do not each have to have the same values in all the strategies. The speeds v₁ to v₅ can also depend on further parameters which characterize the respective driving situation. The strategies can each be configured in such a way that in the case of a velocity which is equal to a maximum speed or minimum speed, the driving mode which is assigned to the relatively low or to the relatively high velocity is selected. The speed at which another driving mode is selected can also be different in the case of a transition in the direction of a relatively high speed than in the case of a transition in the direction of a relatively low speed.

When the brake pedal is actuated, a driving mode (not illustrated in FIG. 1), for example, an active deceleration mode which corresponds to the passive deceleration mode but includes an additional braking effect by the service brake, can be respectively selected. The selected driving mode can also depend on the current speed and, if appropriate, on further parameters of the vehicle and on the driving situation. In addition, the driver can, in particular by actuating the gas pedal, bring about a change into the normal driving mode (not illustrated in FIG. 1 either) in which the motor vehicle is accelerated, for example, by a driving force which is generated in accordance with the position of the gas pedal, or is kept at a constant velocity. 

We claim:
 1. A method of operating a vehicle powertrain, comprising: sensing a vehicle velocity; selecting at least two of a plurality of control strategies; activating the at least two control strategies, said two control strategies including: (i) operating the vehicle in a stationary start-stop mode when the vehicle speed is below a first threshold; and (ii) operating the vehicle in a rolling stationary start-stop mode when the vehicle speed is above a second threshold.
 2. The method of claim 1, wherein one of the at least two control strategies includes: (iii) operating the vehicle in a passive deceleration fuel shut-off mode when the vehicle speed is above a third threshold.
 3. The method of claim 2, wherein the first and third thresholds are equal.
 4. The method of claim 2, wherein one of the at least two control strategies includes: (iv) operating the vehicle in a sailing mode when the vehicle speed is below the second predetermined threshold and above a fourth threshold.
 5. The method of claim 4, wherein one of the at least two control strategies includes: (iv) operating the vehicle in a creeping mode when the vehicle speed is below the third predetermined threshold and above the first threshold.
 6. The method of claim 1, wherein one of the at least two control strategies includes: (iii) operating the vehicle in a sailing mode when the vehicle speed is below the second predetermined threshold and above a third threshold.
 7. The method of claim 6, wherein the first threshold and third threshold are equal.
 8. The method of claim 6, wherein one of the at least two control strategies includes: (iv) operating the vehicle in a rolling stationary start-stop mode when the vehicle speed is above the first threshold and below the third threshold.
 9. The method of claim 6, wherein one of the at least two control strategies includes: (iv) operating the vehicle in a creeping mode when the vehicle speed is above the first threshold and below the third threshold.
 10. A method for operating a motor vehicle having an internal combustion engine, comprising: sensing a velocity of the motor vehicle; sensing actuation of a speed control means; selecting a coasting mode above a minimum speed of said coasting mode when the speed control means is not actuated; and selecting a rolling mode above a maximum speed of the coasting mode when the speed control means is not actuated.
 11. The method of claim 10, further comprising: selecting a stationary mode below a maximum speed of said stationary mode.
 12. The method of claim 11, further comprising: connecting an electric starter to the internal combustion engine when in the stationary mode or in the rolling mode.
 13. The method of claim 10, further comprising: selecting a creeping mode in which there is frictional engagement between the engine and at least one driven wheel of the motor vehicle when above the maximum speed of the stationary mode and below a minimum speed of the coasting mode.
 14. The method of claim 10, further comprising: selecting a rolling mode when above the maximum speed of the stationary mode and below a minimum speed of the coasting mode.
 15. The method of claim 10, further comprising: selecting a passive deceleration mode when above the maximum speed of the stationary mode and below a minimum speed of the coasting mode and when the speed control means is not actuated; wherein in the deceleration mode there is frictional engagement between the drive motor and at least one driven wheel of the motor vehicle, and the supply of fuel to the drive motor is interrupted.
 16. The method of claim 15, further comprising: selecting a creeping mode when below a minimum speed of the passive deceleration mode and above the maximum speed of the stationary mode; wherein in the creeping mode there is frictional engagement between the drive motor and at least one driven wheel of the motor vehicle.
 17. The method of claim 10, wherein selecting a drive mode is done by actuating a pedal brake.
 18. A drive system of a motor vehicle having a drive motor, comprising: a drive train for transmitting driving force to at least one driven wheel of the motor vehicle; and a control device for controlling the drive motor and an automatically actuable clutch, wherein the control device is designed to: (i) select a coasting mode above a minimum speed of said coasting mode when a speed control means is not actuated; and (ii) select a rolling mode above a maximum speed of the coasting mode when the speed control means is not actuated.
 19. The drive system of claim 18, wherein the control device is further configured to select a stationary mode below a maximum speed of said stationary mode.
 20. The drive system of claim 18, wherein the control device is further configured to select a creeping mode, in which there is frictional engagement between the drive motor and at least one driven wheel of the motor vehicle, when above the maximum speed of the stationary mode and below a minimum speed of the coasting mode.
 21. The drive system of claim 18, wherein the control device is further configured to select a rolling mode when above the maximum speed of the stationary mode and below a minimum speed of the coasting mode.
 22. The drive system of claim 18, wherein the control device is further configured to select a passive deceleration mode when above the maximum speed of the stationary mode and below a minimum speed of the coasting mode and when the speed control means is not actuated. 